WO2012/077450A discloses an internal resistance measuring apparatus that can measure an internal resistance of a fuel cell in a state where an electric power is supplied from the fuel cell to a load. In order to secure measurement accuracy, this internal resistance measuring apparatus outputs an identical AC signal to a positive electrode terminal and a negative electrode terminal of the fuel cell so as to avoid a leakage of a current to a load side. The internal resistance measuring apparatus adjusts amplitude (or magnitude) of each of the AC signals outputted to the respective electrode terminals so that a positive-electrode side AC potential difference, which is obtained by subtracting an electric potential of a middle terminal positioned between the positive electrode terminal and the negative electrode terminal from an electric potential of the positive electrode terminal of the fuel cell, matches a negative-electrode side AC potential difference, which is obtained by subtracting the electric potential described above of the middle terminal from an electric potential of the negative electrode terminal. The internal resistance measuring apparatus then measures an internal impedance of the fuel cell on the basis of the adjusted electric potential difference signal and an AC output signal.